Vinylbenzyl Chloride/Styrene-Grafted SBS Copolymers via TEMPO-Mediated Polymerization for the Fabrication of Anion Exchange Membranes for Water Electrolysis

Abstract

In this work, a commercial SBS was functionalized with the 2,2,6,6-tetramethylpiperidin-N-oxyl stable radical (TEMPO) via free-radical activation initiated with benzoyl peroxide (BPO). The obtained macroinitiator was used to graft both vinylbenzyl chloride (VBC) and styrene/VBC random copolymer chains from SBS to create g-VBC-x and g-VBC-x-co-Sty-z graft copolymers, respectively. The controlled nature of the polymerization as well as the use of a solvent allowed us to reduce the extent of the formation of the unwanted, non-grafted (co)polymer, thereby facilitating the graft copolymer’s purification. The obtained graft copolymers were used to prepare films via solution casting using chloroform. The –CH2Cl functional groups of the VBC grafts were then quantitatively converted to –CH2(CH3)3N+ quaternary ammonium groups via reaction with trimethylamine directly on the films, and the films, therefore, were investigated as anion exchange membranes (AEMs) for potential application in a water electrolyzer (WE). The membranes were extensively characterized to assess their thermal, mechanical, and ex situ electrochemical properties. They generally presented ionic conductivity comparable to or higher than that of a commercial benchmark as well as higher water uptake and hydrogen permeability. Interestingly, the styrene/VBC-grafted copolymer was found to be more mechanically resistant than the corresponding graft copolymer not containing the styrene component. For this reason, the copolymer g-VBC-5-co-Sty-16-Q with the best balance of mechanical, water uptake, and electrochemical properties was selected for a single-cell test in an AEM-WE.

Polyolefin‐Based Anion Exchange Membranes with Superior Performance and Long‐Term Durability for Green Hydrogen Production

“…In our work, solution polymerization was preferred to the in‐bulk grafting reaction already reported in the literature, [ 31 ] because the solution polymerization was found to lead to a much more reduced amount of unwanted poly(VBC) homopolymer. [ 30 ] Moreover, the homopolymer produced in solution polymerization was characterized by lower molecular weights and therefore was easier to be effectively removed without the use of time‐, energy‐ and solvent‐consuming hot extractions of the crude product.…”

“…g‐VBC‐ x films were quaternized by immersion in a solution of TMA in methanol (2.5% w/v) at room temperature for 120 h ( Figure ). [ 30 ]…”

“…[ 25–29 ] On the other hand, when starting from an unsaturated copolymer, like poly(styrene‐ b ‐butadiene‐ b‐ styrene) (SBS) linear triblock copolymer, it is possible to take advantage of the reactivity of the double bonds of the polybutadiene block to graft a suitable functionalized monomer to the polymer backbone by the use of milder conditions and without ionizing radiations. [ 30–32 ] By taking advantage of this type of reactivity, AEM copolymer precursors were previously reported to be prepared by in‐bulk post‐functionalization of SBS with VBC through radical grafting. Although interesting, this approach had the great limitation of the high amount of p(VBC) homopolymer formed as byproduct and the difficulty in effectively removing it.…”

“…Moreover, results demonstrate that an increased content of styrene in the SBS‐based grafted copolymer played a key role in improving the AEM mechanical performance, thus ensuring the resistance of the membrane during membrane‐electrode‐assembly (MEA) and the conditioning stage. [ 30 ] Starting from this rationale, here we reported an even simplified synthetic procedure for the preparation of VBC‐grafted copolymers by free (instead of TEMPO‐controlled) radical solution polymerization, starting from a commercially available poly(styrene‐ b ‐butadiene) (SB) block copolymer containing a high content of styrene (64 mol%) with the aim of preparing mechanically robust membranes even at high VBC functionalization degrees (FDs). The VBC‐grafted SB copolymers were then casted from the solution and subjected to heterogeneous quaternization reaction with TMA to convert the films in AEMs.…”

Recent progress in anion exchange membranes (AEMs) in water electrolysis: synthesis, physio-chemical analysis, properties, and applications

“…, g-VBC-5- co -Sty-16-Q). 344 This structured copolymer membrane had poor ionic conductivity (5.7 mS cm −2 ) and maybe low alkaline stability, resulting in a moderate current density and short durability (stable for only 3 h). Therefore, an additional study on membrane modification is required to increase the conductivity and alkaline stability of the membranes in light of AEMWE performances.…”